Micrographia, by Robert Hooke

Observ. xiii. Of the small Diamants, or Sparks in Flints.

Chancing to break a Flint stone in pieces, I found within it a certain cavity all crusted over with
a very pretty candied substance, some of the parts of which, upon changing the posture of the Stone, in respect of the
Incident light, exhibited a number of small, but very vivid reflections; and having made use of my
Microscope, I could perceive the whole surface of that cavity to be all beset with a multitude of little
Crystaline or Adamantine bodies, so curiously shap'd, that it afforded a not unpleasing object.

Having considered those vivid repercussions of light, I found them to be made partly from the plain
external surface of these regularly figured bodies (which afforded the vivid reflexions) and partly to be made from
within the somewhat pellucid body, that is, from some surface of the body, opposite to that superficies of it
which was next the eye.

And because these bodies were so small, that I could not well come to make Experiments and Examinations of them, I
provided me several small stiriæ of Crystals or Diamants, found in great quantities in Cornwall and
are therefore commonly called Cornish Diamants: these being very pellucid, and growing in a hollow
cavity of a Rock (as I have been several times informed by those that have observ'd them) much after the same manner as
these do in the Flint, and having besides their outward surface very regularly shap'd, retaining very near the same
Figures with some of those I observ'd in the other, became a convenient help to me for the Examination of the
proprieties of those kinds of bodies.

And first for the Reflections, in these I found it very observable, That the brightest reflections of light
proceeded from within the pellucid body; that is, that the Rays admitted through the pellucid
substance in their getting out on the opposite side, were by the contiguous and strong reflecting surface of the Air
very vividly reflected, so that more Rays were reflected to the eye by this surface, though the Ray in entring and
getting out of the Crystal had suffer'd a double refraction, than there were from the outward surface of the Glass
where the Ray had suffer'd no reflection at all.

And that this was the surface of the Air that gave so vivid a re-percussion I try'd by this means I sunk
half of a stiria in Water, so that only Water was contiguous to the under surface, and then the internal
reflection was so exceedingly faint, that it was scarce discernable. Again, I try'd to alter this vivid reflection by
keeping off the Air, with a body not fluid, and that was by rubbing and holding my finger very hard against the under
surface, so as in many places the pulp of my finger did touch the Glass, without any interjacent air between,
then observing the reflection, I found, that wheresoever my finger or skin toucht the surface, from that part there was
no reflection, but in the little furrows or creases of my skin, where there remain'd little small lines of air, from
them was return'd a very vivid reflection as before. I try'd further, by making the surface of very pure Quicksilver to
be contiguous to the under surface of this pellucid body, and then the reflection from that was so exceedingly
more vivid than from the air, as the reflection from air was than the reflection from the Water; from all which trials
I plainly saw, that the strong reflecting air was the cause of this Phænomenon.

And this agrees very well with the Hypothesis of light and Pellucid bodies which I have mention'd
in the description of Muscovy-glass; for we there suppose Glass to be a medium, which does less
resist the pulse of light, and consequently, that most of the Rays incident on it enter into it, and are refracted
towards the perpendicular; whereas the air I suppose to be a body that does more resist it, and consequently
more are re-percuss'd then do enter it: the same kind of trials have I made, with Crystalline Glass,
with drops of fluid bodies, and several other ways, which do all seem to agree very exactly with this Theory.
So that from this Principle well establish'd, we may deduce severall Corollaries not unworthy observation.

And the first is; that it plainly appears by this, that the production of the Rainbow is as much to be ascribed to
the reflection of the concave surface of the air, as to the refraction of the Globular drops: this will be
evidently manifest by these Experiments, if you foliate that part of a Glass-ball that is to reflect an
Iris, as in the Cartesian Experiment, above mention'd, the reflections will be abundantly more
strong, and the colours more vivid: and if that part of the surface be touch'd with Water, scarce affords any sensible
colour at all.

Next we learn, that the great reason why pellucid bodies beaten small are white, is from the multitude of
reflections, not from the particles of the body, but from the contiguous surface of the air. And this is
evidently manifested, by filling the Interstitia of those powder'd bodies with Water, whereby their whiteness
presently disappears. From the same reason proceeds the whiteness of many kinds of Sands, which in the
Microscope appear to be made up of a multitude of little pellucid bodies, whose brightest reflections
may by the Microscope be plainly perceiv'd to come from their internal surfaces; and much of the whiteness of
it may be destroy'd by the affusion of fair Water to be contiguous to those surfaces.

The whiteness also of froth, is for the most part to be ascribed to the reflection of the light from the surface of
the air within the Bubbles, and very little to the reflection from the surface of the Water it self: for this last
reflection does not return a quarter so many Rays, as that which is made from the surface of the air, as I have
certainly found by a multitude of Observations and Experiments.

The whiteness of Linnen, Paper, Silk, &c. proceeds much from the same reason, as the
Microscope will easily discover; for the Paper is made up of an abundance of pellucid bodies, which
afford a very plentifull reflection from within, that is, from the concave surface of the air contiguous to its
component particles; wherefore by the affusion of Water, Oyl, Tallow, Turpentine, &c. all those
reflections are made more faint, and the beams of light are suffer'd to traject & run through the Paper more
freely.

Hence further we may learn the reason of the whiteness of many bodies, and by what means they maybe in part made
pellucid: As white Marble for instance, for this body is composed of a pellucid body exceedingly
flaw'd, that is, there are abundance of thin, and very fine cracks or chinks amongst the multitude of particles of the
body, that contain in them small parcels of air, which do so re-percuss and drive back the penetrating beams,
that they cannot enter very deep within that body; which the Microscope does plainly inform us to be made up
of a Congeries of pellucid particles. And I further found it somewhat more evidently by some attempts
I made towards the making transparent Marble, for by heating the Stone a little, and baking it in Oyl, Turpentine, Oyl
of Turpentine, &c., I found that I was able to see much deeper into the body of Marble then before; and
one trial, which was not with an unctuous substance, succeeded better than the rest, of which, when I have a better
opportunity, I shall make further trial.

This also gives us a probable reason of the so much admired Phænomena, of the Oculus Mundi, an
Oval stone, which commonly looks like white Alabaster, but being laid a certain time in Water, it grows
pellucid, and transparent, and being suffer'd to lie again dry, it by degrees loses that transparency, and
becomes white as before. For the Stone being of a hollow spongie nature, has in the first and last of these
appearances, all those pores fill'd with the obtunding and reflecting air; whereas in the second, all those pores are
fill'd with a medium that has much the same refraction with the particles of the Stone, and therefore those
two being contiguous, make, as 'twere, one continued medium, of which more is said in the 15.
Observation.

There are a multitude of other Phænomena, that are produc'd from this same Principle, which as it has not
been taken notice of by any yet that I know, so I think, upon more diligent observation, will it not be found the least
considerable. But I have here onely time to hint Hypotheses, and not to prosecute them so fully as I could
wish; many of them having a vast extent in the production of a multitude of Phænomena, which have been by
others, either not attempted to be explain'd, or else attributed to some other cause than what I have assign'd, and
perhaps than the right; and therefore I shall leave this to the prosecution of such as have more leisure: onely before
I leave it, I must not pretermit to hint, that by this Principle, multitudes of the Phænomena of the air, as
about Mists, Clouds, Meteors, Haloes, &c. are most plainly and (perhaps) truly
explicable; multitudes also of the Phænomena in colour'd bodies, as liquors, &c. are deducible
from it.

And from this I shall proceed to a second considerable Phænomenon which these Diamants exhibit, and that is
the regularity of their Figure, which is a propriety not less general than the former, It comprising within
its extent, all kinds of Metals, all kinds of Minerals, most Precious stones, all kinds of
Salts, multitudes of Earths, and almost all kinds of fluid bodies. And this is another
propiety, which, though a little superficially taken notice of by some, has not, that I know, been so much as attempted
to be explicated by any.

This propriety of bodies, as I think it the most worthy, and next in order to be consider'd after the contemplation
of the Globular Figure, so have I long had a desire as wel as a determination to have prosecuted it if I had
had an opportunity, having long since propos'd to my self the method of my enquiry therein, it containing all the
allurements that I think any enquiry is capable of: For, first I take it to proceed from the most simple principle that
any kind of form can come from, next the Globular, which was therefore the first I set upon, and what I have
therein perform'd, I leave the Judicious Reader to determine. For as that form proceeded from a propiety of fluid
bodies, which I have call'd Congruity, or Incongruity; so I think, had I time and opportunity, I
could make probable, that all these regular Figures that are so conspicuously various and curious,
and do so adorn and beautifie such multitudes of bodies, as I have above hinted, arise onely from three or four several
positions or postures of Globular particles, and those the most plain, obvious, and necessary conjunctions of
such figur'd particles that are possible, so that supposing such and such plain and obvious causes concurring the
coagulating particles must necessarily compose a body of such a determinate regular Figure, and no other, and
this with as much necessity and obviousness as a fluid body encompast with a Heterogeneous fluid must be
protruded into a Spherule or Globe. And this I have ad oculum demonstrated with a company of
bullets, and some few other very simple bodies; so that there was not any regular Figure, which I have hitherto met
withall, of any of those bodies that I have above named, that I could not with the composition of bullets or globules,
and one or two other bodies, imitate, even almost by shaking them together. And thus for instance may we find that the
Globular bullets will of themselves, if put on an inclining plain, so that they may run together, naturally
run into a triangular order, composing all the variety of figures that can be imagin'd to be made out of
æquilateral triangles; and such will you find, upon trial, all the Surfaces of Alum to be compos'd
of: For three bullets lying on a plain, as close to one another as they can compose an Schem. 7.Fig. A. &c.æquilatero-triangular form, as in A in the 7. Scheme. If a
fourth be joyn'd to them on either side as closely as it can, they four compose the most regular Rhombus consisting of
two æquilateral triangles, as B. If a fifth be joyn'd to them on either side in as close a position as it can,
which is the propriety of the Texture, it makes a Trapezium, or four-sided Figure, two of whole
angles are 120. and two 60. degrees, as C. If a sixth be added, as before, either it makes an æquilateral
triangle, as D, or a Rhomboeid, as E, or an Hex-angular Figure, as F, which is compos'd of two
primary Rhombes. If a seventh be added, it makes either an æquilatero-hexagonal Figure, as G, or some
kind of six-sided Figure, as H, or I. And though there be never so many placed together, they may be rang'd
into some of these lately mentioned Figures, all the angles of which will be either 60. degrees, or 120. as
the figure K. which is an æquiangular hexagonal Figure is compounded of 12. Globules, or may be of
25, or 27, or 36, or 42, &c. and by these kinds of texture, or position of globular bodies, may you find
out all the variety of regular shapes, into which the smooth surfaces of Alum are form'd, as upon examination
any one may easily find; nor does it hold only in superficies, but in solidity also, for it's obvious that a fourth
Globule laid upon the third in this texture, composes a regular Tetrahedron, which is a very usual
Figure of the Crystals of Alum. And (to hasten) there is no one Figure into which Alum is
observ'd to be crystallized, but may by this texture of Globules be imitated, and by no other.

I could instance also in the Figure of Sea-salt, and Sal-gem, that it is compos'd of a texture of
Globules, placed in a cubical form, as L, and that all the Figures of those Salts may be imitated by
this texture of Globules and by no other whatsoever. And that the forms of Vitriol and of
Salt-Peter, as also of Crystal, Hore-frost, &c. are compounded of these two textures,
but modulated by certain proprieties: But I have not here time to insist upon, as I have not neither to shew by what
means Globules come to be thus context, and what those Globules are, and many other particulars
requisite to a full and intelligible explication of this propriety of bodies. Nor have I hitherto found indeed an
opportunity of prosecuting the inquiry so farr as I design'd; nor do I know when I may, it requiring abundance of time,
and a great deal of assistance to go through with what I design'd; the model of which was this:

First, to get as exact and full a collection as I could, of all the differing kinds of Geometrical figur'd bodies,
some three or four several bodies of each kind.

Secondly, with them to get as exact a History as possibly I could learn of their places of Generation or finding,
and to enquire after as many circumstances that tended to the Illustrating of this Enquiry, as possibly I could
observe.

Thirdly, to make as many trials as upon experience I could find requisite, in Dissolutions and Coagulations of
several crystallizing Salts; for the needfull instruction and information in this Enquiry.

Fourthly, to make several trials on divers other bodies, as Metals, Minerals, and Stones, by dissolving them in
several Menstruums, and crystalizing them, to see what Figures would arise from those several
Compositums.

Fifthly, to make Compositions and Coagulations of several Salts together into the same mass, to observe of what
Figure the product of them would be; and in all, to note as many circumstances as I should judge conducive to my
Enquiry.

Sixthly, to enquire the closeness or rarity of the texture of these bodies, by examining their gravity, and their
refraction, &c.

Seventhly, to enquire particularly what operations the fire has upon several kinds of Salts, what changes it causes
in their Figures, Textures, or Energies.

Eighthly, to examine their manner of dissolution, or acting upon those bodies dissoluble in them; The texture of
those bodies before and after the process. And this for the History.

Next for the Solution, To have examin'd by what, and how many means, such and such Figures, actions and effects
could be produc'd possibly.

And lastly, from all circumstances well weigh'd, I should have endeavoured to have shewn which of them was most
likely, and (if the informations by these Enquiries would have born it) to have demonstrated which of them it must be,
and was.

But to proceed, As I believe it next to the Globular the most simple; so do I, in the second place, judge it not
less pleasant; for that which makes an Enquiry pleasant, are, first a noble Inventum that promises to crown
the successfull endeavour; and such must certainly the knowledge of the efficient and concurrent causes of all these
curious Geometrical Figures be, which has made the Philosophers hitherto to conclude nature in these things to play the
Geometrician, according to that saying of Plato, ‛Ο
Θεος γεομετρει. Or next, a great variety of matter in the Enquiry; and here we meet with nothing less than the
Mathematicks of nature, having every day a new Figure to contemplate, or a variation of the same in another
body,

Which do afford us a third thing, which will yet more sweeten the Enquiry, and that is, a multitude of information;
we are not so much to grope in the dark, as in most other Enquiries, where the Inventum is great; for having
such a multitude of instances to compare, and such easie ways of generating, or compounding and of destroying the form,
as in the Solution and Crystallization of Salts, we cannot but learn plentifull information to
proceed by. And this will further appear from the universality of the Principle which Nature has made use of almost in
all inanimate bodies. And therefore, as the contemplation of them all conduces to the knowledg of any one; so from a
Scientifical knowledge of any one does follow the fame of all, and every one.

And fourthly, for the usefulness of this knowledge, when acquir'd; certainly none can doubt, that considers that it
caries us a step forward into the Labirinth of Nature, in the right way towards the end we propose our selves in all
Philosophical Enquiries. So that knowing what is the form of Inanimate or Mineral bodies, we shall be the better able
to proceed in our next Enquiry after the forms of Vegetative bodies; and last of all, of Animate ones, that seeming to
be the highest step of natural knowledge that the mind of man is capable of.